Hi All. I was recently at a home show and when I passed the spa dealer a coustomer looked like he was having a problem deciding weather to get a 220v unit or a 115v unit. I quickly jumped at the oportunity to tell him that 220v would be cheaper but I could not remember how to explain it. The spa salesman replied that what I was saying was not true and that they have done studies on which is cheaper to run and the 115v unit would cost less to run. Now I am doubting my knowledge. Can any one explain this to me. Am I right?
is 220 cheaper than 115
- Thread starter bnb
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quogueelectric
Senior Member
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- new york
Watts is watts he may be right.
A 120V unit is most likely going to have a smaller heater. A typical hot tub is 50A at 240V. A 120V one is not going to have a 100A branch circuit. Therefore, the 120V one must draw less current and will be cheaper to run. But since it draws less power, it also will heat slower and may not pump as much water (weaker pump).
If everything was equal (power use), then the cost differences between 120V or 240V is negligible. Kind of like heating a house with a 120V space heater -vs- a 20 KW 240V furnace. The space heater will be cheaper to run, but only one room is going to be warm in the house.
If everything was equal (power use), then the cost differences between 120V or 240V is negligible. Kind of like heating a house with a 120V space heater -vs- a 20 KW 240V furnace. The space heater will be cheaper to run, but only one room is going to be warm in the house.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
There is an argument to make for 240V being a little less expensive to operate (besides the transmission losses). It depends on the size / power rating of the spa and how you use it.
To illustrate it, I'll use my spa. I ran a 50A 240V circuit to it. When the heater is on and the pump is running full out (it has a 2 speed motor), I have measured the amp draw at about 40A. If that were a 120V circuit, it would be 80A so it would need to be a 100A circuit. If I'm not mistaken, I don't think you can buy a 100A 1 pole breaker; I think 70A is the biggest. So if it were 120V, they would need to use a smaller heater element. If the heater element is smaller, it will still heat up the water, but it will take longer. So technically, the same net amount of kWH would be used to heat up the same amount of water to the same temperature.
BUT, you also have to look at how the spa is used. I do not keep mine at full temperature all the time. I keep it as low as it will go (80F) and turn it up in advance of using it. So for my purposes, I want it to heat up as fast as it can.
The final step in this is to know how spas work as well. For most spas, if the heater element is on, the pump MUST be on with it, otherwise you damage the heater element. So if the heater takes longer to heat up the water, the pump is going to run longer as well, which WILL make it consume more energy if run at 120V. To be fair though, if you ALWAYS keep it on at full temperature, that will not be as much of an issue because maintaining temperature is a lot different than raising temperature.
To illustrate it, I'll use my spa. I ran a 50A 240V circuit to it. When the heater is on and the pump is running full out (it has a 2 speed motor), I have measured the amp draw at about 40A. If that were a 120V circuit, it would be 80A so it would need to be a 100A circuit. If I'm not mistaken, I don't think you can buy a 100A 1 pole breaker; I think 70A is the biggest. So if it were 120V, they would need to use a smaller heater element. If the heater element is smaller, it will still heat up the water, but it will take longer. So technically, the same net amount of kWH would be used to heat up the same amount of water to the same temperature.
BUT, you also have to look at how the spa is used. I do not keep mine at full temperature all the time. I keep it as low as it will go (80F) and turn it up in advance of using it. So for my purposes, I want it to heat up as fast as it can.
The final step in this is to know how spas work as well. For most spas, if the heater element is on, the pump MUST be on with it, otherwise you damage the heater element. So if the heater takes longer to heat up the water, the pump is going to run longer as well, which WILL make it consume more energy if run at 120V. To be fair though, if you ALWAYS keep it on at full temperature, that will not be as much of an issue because maintaining temperature is a lot different than raising temperature.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Our posts crossed in the ether. We basically agree, but...
We don't pay for current, we pay for watt-hours. Watts are watts are watts. Less current x more time = same kWH as more current x less time.
Natfuelbilll
Senior Member
If the utility installs a demand meter, how the demand be recorded?
My guess is that demand ratchets up on the highest kW by phase, and if I'm right, then the 120V unit would be more costly due to higher demand billings.
My guess is that demand ratchets up on the highest kW by phase, and if I'm right, then the 120V unit would be more costly due to higher demand billings.
- Location
- Massachusetts
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I do not believe the power companies install demand meters on dwelling units.
Watts are watts, all other things remaining the same the 240 unit may be cheaper to install due to smaller circuits.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I once installed a Jacuzzi that the heater ran on either 240 or 120 volts. The 120 volt hookup had 1/4 the power output of the 240 so yes it would use less energy but would heat the water at a much slower rate.
- Location
- Illinois
- Occupation
- retired electrician
Would you really use less energy? It will take the same amount of BTUs to heat the same amount of water to the same temperature. The actual energy used to heat the water should remain the same.
Ideally yes, in a vacuum insulated bottle perhaps... but in this real world scenario the water not in contact with the heater(s) is cooling at the same time. Hence, at 1/4 power it will take more than 4 times longer to heat the water to the same temperature.
- Location
- Illinois
- Occupation
- retired electrician
How would you use less energy, even in a vacuum insulated bottle? It still takes 1 BTU of energy to heat 1 pound of water 1 degree. I can see the energy use being equal in an ideal closed insulated system, but I don't see any way that operating the heaters at 120 volts will save any energy over operating them at 240 volts.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Everyone is making what I believe to be an invalid assumption. I believe that, all else being equal, and given that "watts is watts," the cost difference should be small. But why should we assume that all else is equal? Perhaps the pump associated with 220V unit has a much higher HP rating. Perhaps the heater associated with the 120V unit has a higher BTU/HR rating. Perhaps the 220V unit has a much larger internal volume (more water to heat and to circulate). We really don't know anything about the characteristics of this manufacturer's product line.
mikeames
Senior Member
- Location
- Gaithersburg MD
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- Teacher - Master Electrician - 2017 NEC
Not the whole system. Just the motor part. Excluding the motor, and (wire resistance) its the same.
How so?
...
Because it is the only basis for making a considered comment?
And these replies were generally qualified to include such phrases as "all other things remaining the same" to provide the basis on which they were made.
If you throw in a whole load of "what ifs" you cant conclude anything.
Well the question is a fair one, and the answer depends on if the load is inductive or resistive. Since we are talking about a resistive heating element I will limit my comments to resistive only.
A simple circuit analysis is needed to find the answer. So lets assume a 1000 watt element, with 12 AWG being used at a one way distance of 100 feet.
At 120 volts the element resistance is 14.4 ohms, and at 240 it is 57.6 ohms. 100 feet of 12 AWG is constant at .2 ohms one way, .4 ohms round trip.
For the 120 VAC circuit we consume a total power of 974 watts, with 945 being delivered to the load, and 26 watts consumed by the copper wire.
For the 240 circuit we consume a total power of 992 watts, with 986 delivered to the load, and 7 watts consumed by the copper wire.
So running at higher voltage uses more total power, but deliver higher efficiency at the load, and less loss on the wiring. So I can assume running at higher voltage would cost less to operate because because I deliver more heat to the load for a shorter period of time.
A simple circuit analysis is needed to find the answer. So lets assume a 1000 watt element, with 12 AWG being used at a one way distance of 100 feet.
At 120 volts the element resistance is 14.4 ohms, and at 240 it is 57.6 ohms. 100 feet of 12 AWG is constant at .2 ohms one way, .4 ohms round trip.
For the 120 VAC circuit we consume a total power of 974 watts, with 945 being delivered to the load, and 26 watts consumed by the copper wire.
For the 240 circuit we consume a total power of 992 watts, with 986 delivered to the load, and 7 watts consumed by the copper wire.
So running at higher voltage uses more total power, but deliver higher efficiency at the load, and less loss on the wiring. So I can assume running at higher voltage would cost less to operate because because I deliver more heat to the load for a shorter period of time.
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